1. Field of the Invention
The present invention relates to a data display apparatus for displaying input data on a CRT (Cathode-Ray Tube) having format such as scales and characters preformed on a screen, and more particularly, to a data display apparatus with a function for correcting the effect of external magnetic field such as geomagnetism.
2. Description of the Prior Art
A CRT used in a data display apparatus produces images such as waveforms, figures, characters or the like on a fluorescent screen by hitting the screen with an electron beam.
A digital oscilloscope and a logic analyzer are examples of the apparatuses that use CRTs. A major function of a digital oscilloscope is to perform analog-to-digital conversion on an input signal so as to store the converted digital data into a memory, and to perform digital-to-analog conversion on the stored digital data in order to display the converted analog signal on the screen of the CRT as an image of the input signal.
The electron beam of the CRT, however, is subjected to the effect of external magnetic field such as geomagnetism. Accordingly, changes in positional relationship between the digital oscilloscope and the external magnetic field alter positions of the waveform of the input signal on the screen of the CRT. The scales fixed on the screen, however, are not subjected to the effect of the external magnetic field. Consequently, the waveform of the input signal is displaced with regard to the format on the screen by the effect of the external magnetic field.
Some digital oscilloscopes have a function to transfer the stored data in the memory to an external hardcopy device such as a plotter or a printer via an interface. Such a digital oscilloscope can also transmit the following stored data to an external hardcopy device: stored input data; stored format data which correspond to scales and characters preformed on the screen of the CRT by printing or the like; and stored data representing characters and a cursor to be drawn on the screen by an electronic beam. Thus, the format data such as scales are stored in the memory beforehand, and are transferred together with the stored input data to the external hardcopy device. On the basis of these data, the external device reproduces the image similar to that displayed on the CRT of a digital oscilloscope or the like.
In such apparatuses, when displacement of waveforms of input signals is induced by the effect of external magnetic field, the image displayed on the screen of the CRT of the digital oscilloscope or the like and the image reproduced by the external device disagree. This is because the image reproduced by the external device maintains correct position because the stored data and format data in the memory are not affected by the external magnetic field.
For this reason, a conventional data display apparatus eliminates the effect of the external magnetic field by providing the data display apparatus with a magnetic shield so that the image displayed on the data display apparatus and the image reproduced on the external device will coincide.
Another technique is proposed which displays format data together with the input signal data on the screen of the CRT by using the electron beam instead of providing the preformed format on the screen of the CRT. This technique can prevent the relative displacement between the images of the input signal and format because these two images are equally displaced by the external magnetic field.
However, the first method of providing the magnetic shield presents problems in that it is technically difficult to perform a complete magnetic shield, and that the apparatus becomes costly because magnetic shield materials are expensive. In addition, achieving a complete magnetic shield will make the apparatus heavier and larger.
The second method of displaying the format together with the input signal presents problems in that the amount of display increases by the amount of displaying the format. In particular, the apparatus employing an electrostatic deflection CRT has a problem in that the image flicker occurs because of increased update duration of the image. Furthermore, this method requires a position calibration instrument for correcting the position of an entire image which must be uniformly displaced.